This invention relates to digital transmission systems; in particular, it relates to the transmission of signals requiring more bandwidth than the typical signals for which the transmission system is primarily designed.
One situation requiring such broader bandwidth is the transmission of a music program signal over T carrier facilities. Studios and concert halls where the programs originate are often many miles from the transmitter. To design and maintain a custom transmission line to connect the studio and transmitter is expensive, and often prohibitively so. Broadband analog facilities capable of providing the service exist in the telephone network, however, usually only over heavy traffic routes. T carrier digital facilities, on the other hand, are becoming ubiquitous. T carrier is a pulse code modulated system (PCM) based upon 8-bit words and 8 kHz sampling. This allows faithful reproduction of 255 amplitude levels, and according to the Nyquist theorem, signal frequencies of 4 kHz bandwidth. A signal T1 line can carry 24 time-division-multiplexed channels of this definition. Understandably, this combination of levels and bandwidth per channel is considered inadequate for music channels. Nevertheless, more than adequate bandwidth is recognizably available if more than one channel space is used; there are, however, other restrictions. In order to insure timing recovery for the extant T1 repeaters, the average density of pulses, or "1"s must exceed one-out-of-eight bits and the signal must never consist of more than fifteen consecutive spaces or "zeros". These requirements are normally met by the standard .mu.255 encoding with zero-code suppression which prevents the transmission of an 8-bit all zeros code word.
In the PC-79 digital studio link, manufactured by Bayly Engineering, Limited, Ajax, Ontario, Canada, and described in a brochure entitled "An Introduction to Digital Studio Transmission Links" the music is sampled at a 32 kHz rate. The samples are converted to 12-bit digital words, and four of the 12-bit words are inserted into six adjacent 8-bit time slots in a T carrier system. While this link functions to transmit a 15 kHz audio signal over the digital telephone network, it is less than optimal. In order to meet the timing recovery requirements, the eleventh bit of each 12-bit word is made a "1", and the twelfth bit is an even parity bit, guaranteeing at least two "1"s in every 12-bit word. That, however, leaves only ten bits for encoding the samples, i.e., 1,024 amplitude levels.
An object of this invention is an improved arrangement for transmitting broadband signals via digital facilities of limited bandwidth.
A second object is a digital audio program link for T carrier that makes more efficient use of the coding algorithm to satisfy timing recovery.
A third object is a digital program link that does not require a specific bit in every word to be a "1".
A fourth object of this invention is an improved digital studio link that uses eleven bits for encoding samples.